The overall objective of this project is to delineate the role of properdin and the Properdin Activator System in the activation of complement. The proposed work aims at further definition of C3NeF and its relationship to C1, properdin convertase, and properdin. In addition, this proposal aims at delineating the role of properdin convertase activated properdin with factors B and D. Further, this protocol hopes to characterize the C3 inhibitor in normal human serum and show its relationship, if any, to properdin. Inherent in this latter objective is the determination of the conditions which predispose properdin to act as an activator or inhibitor of the classical and alternative pathways. Finally, it is hoped to demonstrate the mechanism of properdin activation in patients with vasculitis.